Landing: a66820ce3f5b

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Project / Subsystem

gcc / tree-optimization

Date

2026-04-09

Author

Philipp Tomsich

Commit

a66820ce3f5b298759042b6543a056b490370310

Source

github

Perf win

Yes

Breaking

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All attributes

project: gcc
subsystem: tree-optimization
patch_id: —
commit_hash: a66820ce3f5b298759042b6543a056b490370310
source_type: github
headline: Tree-optimization: Avoid Shift-by-64 Undefined Behavior
tldr: The DeBruijn CLZ table validator now avoids undefined behavior by special-casing the MSB bit, resolving a long-standing issue.
author: Philipp Tomsich
outcome: committed
performance_win: true
breaking_change: false
series_id: —
series_parts: []
tags: • tree-optimization
• undefined behavior
• count leading zeros
• DeBruijn
discussion_id_link: —
bugzilla_pr: —
url: https://github.com/gcc-mirror/gcc/commit/a66820ce3f5b298759042b6543a056b490370310
date: 2026-04-09T00:00:00.000Z

The DeBruijn CLZ (count leading zeros) table validator in GCC’s tree-optimization pass had an issue where it would perform a shift by the number of bits in a HOST_WIDE_INT, resulting in undefined behavior. This affected 64-bit inputs, causing the validator to reject well-formed CLZ tables. The fix special-cases this scenario, ensuring correct validation and enabling optimizations relying on DeBruijn CLZ tables.